The development in the global economy is bringing along a continuous increase in the demand for light olefins around the world. It was predicted that the total global demand for ethylene and propylene will be 140 Mt/a and 86 Mt/a, respectively by year of 2010. In China, due to the rapid increase in the national economy, the annual increase rate in the demand for light olefins is estimated to exceed the world's average level. At present, the increase rate in the demand for propylene exceeds that for ethylene. There are many processes for producing light olefins, wherein a steam-cracking technology using a light feedstock such as naphtha as the hydrocarbon oil feedstock is widely used in the world. More than 90% of the total ethylene and about 70% of the total propylene in the world are produced by the steam-cracking technology using petroleum hydrocarbons as the feedstock. However, this technology can not meet the increasing demand for light olefins. Meanwhile, crude oils become heavier and heavier around the world. The yield to produce light olefins from light hydrocarbons, such as naphtha and straight-run light diesel oil, is generally only about ⅓. Further, China suffers from insufficient supply of fuel oil and low ratio of gasoline/diesel oil output, leading to a severer shortage of light hydrocarbon feedstock. Under this circumstance, the development of a technical route for producing light olefins directly from heavy oil becomes a tendency.
U.S. Pat. No. 5,944,982 and U.S. Pat. No. 6,287,522 disclose a catalytic cracking process and a fluidized catalytic cracking apparatus using a dual riser reactor, wherein heavy feed oil is cracked in a first riser, then introduced to a product fractionating system, further, the separated gasoline or light cycle oil is then introduced to a second riser, wherein deep cracking is further carried out under a very severe condition, so as to produce more light olefins. The two risers of the apparatus share one disengager and one regenerator, while the effluents from the first riser reactor and the second riser reactor enter two fractionating towers, respectively.
CN1118539C discloses a two-stage riser catalytic cracking process, which mainly aims at series oil gas, catalyst relay, sub-section reaction, shortening reaction tilde and enhancing average performance of catalysts by using a two-stage riser reactor.
CN1526794A discloses a catalytic cracking process, which comprises: 1) catalytically cracking a feed oil for less than 1.5 s in a first riser, then introducing the effluent stream to a first fractionator; 2) catalytically cracking the resultant cycle oil from the first fractionator for less than 1.5 s, then introducing the resultant stream to the first fractionator; and 3) catalytically cracking the resultant naphtha (gasoline) and/or optionally the resultant diesel oil from the first fractionator, wherein the reaction conditions in the first to the third risers depend on the catalyst used, which can be determined according to the desired catalytic cracking products.
However, the prior art suffers from the problem that the yield to produce light olefins from feed oil by a catalytic cracking is not so high to meet the increasing demand for light olefins in the industry. Furthermore, a great amount of less valuable dry gas is produced from the catalytic cracking of feed oil. Therefore, how to further enhance the conversion of feed oil to light olefins and simultaneously lower the yield of dry gas remains a problem to be solved in the prior art.